Population status and structure of mountain nyala in the Bale Mountains National Park, Ethiopia

Abstract

The study on the population status and structure of mountain nyala (Tragelaphus buxtoni) was carried out in the Bale Mountains National Park, Ethiopia during the wet and dry seasons of 2000/2001.Total counts were carried out in an area of13.1km 2 . The count ranged between 682 and 732 individuals. Females comprised 62.9% while males 27.4%.The calf ratio was 9.7%.The group structure of the animals varied with season. Large herds were frequently observed during the wet season and smaller groups during the dry season. Group size changed seasonally. In both seasons, 7^12 individuals were the frequent group size formation. The maximum group (62) was observed during the wet season. The population trend of the animal from 1975 to 1990 indicated a recovery and increase. But between 1990 and 1995 the population showed a continuous and steep decline. However, between 1995 and 2000 the population trend showed a better condition when compared with the previous years.

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“…The majority of previous studies on mountain nyala found the sex ratio of females to males to be near 2:1 [2,10,11]. Our estimate of adult sex ratio, while slightly lower, agrees with these reports (1.7:1 or 0.63 as a proportion).…”

“…We observed a group size of 3.7 individuals which is less than half that of the estimates found by Refera and Bekele [10] and Mamo et al [10] which estimated group size in Gaysay and the park headquarters between 7.9-10.2 individuals and 7-12 individuals, respectively. Historical accounts of group size in Gaysay were estimated at an average of 5.6 individuals [2] and groups were rarely observed with more than 12 individuals with most group sizes between 4-12 individuals [32].…”

“…Counts conducted by Mamo et al [11] found that mountain nyala density in Gaysay was 21/km 2 and 129/km 2 at the park headquarters. Although these numbers are alarmingly high when compared to Brown's estimates (1.5/km 2 to 6.9/km 2 ) they are supported by other studies [4,10]. The high density of mountain nyala and confinement to a small area poses a number of risks including the loss of food resources, reduction in productivity and genetic diversity, and potential transmission of disease [23][24][25].…”

“…These estimates are higher than those reported in Gaysay and park headquarters. Mamo et al [10] described productivity estimates for calves to be 0.08 and 0.09 for Gaysay and the park headquarters, respectively. These estimates are over three times lower than the estimates we observed in the Abasheba-Demaro CHA (0.42; Table 4).…”

“…These estimates are over three times lower than the estimates we observed in the Abasheba-Demaro CHA (0.42; Table 4). For juveniles, when calculated by combining both adult females and sub-adult females, Mamo et al [10] show productivity to be 0.19 and 0.17 for Gaysay and the park headquarters, respectively. Our estimates of productivity in the Abasheba-Demaro study area are closer to historical records from Brown [2] who estimated productivity across the Bale Mountains between 0.25-0.33 calves/adult female.…”